1. Field of the Invention
The present invention is directed to a method for magnetic resonance imaging with adherence to SARs(Specific Absorption Rate) limit values of the type, wherein a patient is subjected to a radio-frequency pulse sequence via at least one transmission antenna for the implementation of a measurement in a magnetic resonance tomography apparatus, and magnetic resonance signals that are produced are acquired in a spatially resolved manner via at least one reception antenna and further-processed for producing magnetic resonance images or spectra, with current SAR values being determined before the implementation of the measurement on the basis of patient data and the position of the patient relative to the transmission antenna for planned parameters of the measurement, and wherein the parameters are modified as warranted until the current SAR values lie within the SAR limit values. The invention also is directed to a magnetic resonance system for the implementation of such a method.
2. Description of the Prior Art
Magnetic resonance tomography is a known technique for acquiring images of the inside of the body of an examination subject. For implementation of magnetic resonance tomography, a basic field magnet generates a static, relatively homogeneous basic magnetic field. Rapidly switched gradient fields for location coding that are generated by, gradient coils, are superimposed on this basic magnetic field during the exposure of magnetic resonance images. Sequences of radio-frequency pulses for triggering magnetic resonance signals are emitted into the examination subject with one or more radio-frequency transmission antennas. The magnetic resonance signals produced as a result of these radio-frequency pulses are received by radio-frequency reception antennas. Tomograms of the inside of the body of the patient are calculated and displayed on the basis of the magnetic resonance signals received from the field of view (FoV) under observation, possibly covering one or more body slices of the patient.
All body regions from the head to the foot can be measured in this way by displacement of the patient bed within the magnetic resonance tomography apparatus.
An important requirement in modern magnetic resonance tomography is the capability for fast imaging. This demand results from economic considerations of being able to examine as many patients as possible within a given time interval and, as well as from specific medical questions wherein a fast imaging is required for the examination result. One example of this is the reduction of motion artifacts due to movement of the patient during the measurement.
The high repetition rate of the radio-frequency transmission pulses and transmission pulse sequences required for a fast imaging, however, leads to a higher stress on the patient from electromagnetic radiation. Due to legal regulations, limit values are prescribed for this SAR (SAR=Specific Absorption Rate) stress that cannot be exceeded in magnetic resonance imaging. Since modern magnetic resonance tomography systems are technically capable of stressing patients with significantly higher SAR values, arrangements referred to as SAR monitors must be utilized in order to assure adherence to the limit values in the measurement. In addition to whole-body SAR values, specific limit values also must be adhered to for various body regions, and a fundamental distinction must be made between whole-body, partial body and local exposures.
The SAR stress is dependent on the individual patient data as well as on the position of the patient relative to the transmission antenna, the type of transmission antenna, and the transmission power (which is essentially defined by the type of pulse sequence) the flip angle of the RF pulses employed, the repetition rate, and the number of simultaneously acquired slices. The parameters of the measurement are usually summarized in a measurement protocol.
German OS 40 42 212 discloses a method for magnetic resonance imaging wherein the magnetic primary field is cyclically switched between two field strengths during the implementation of a measurement in order to adhere to the SAR limit values. Further details about the determination of the SAR values are not provided in this publication.
The determination of the SAR values for a particular measurement situation, i.e. for the individual data of the patient, the position of the patient relative to the transmission antenna, the type of transmission antenna or antennas, the radio-frequency pulse power as well as the further measurement parameters such as repetition rate or number of slices to be measured, conventionally has ensued at the beginning of the actual measurement. Due to the numerous influencing parameters, a very rough simulation model is utilized for the calculation of the current SAR values in order to keep the time expenditure for the calculation within limits. The application of a very rough simulation model, however, requires that correspondingly large tolerances be taken into consideration.
German OS 198 29 640 discloses a method for the implementation of an image-based diagnosis with which the cause or alleviating measures for artifacts that occur in the images of an imaging facility such as, for example, an MR tomography apparatus are identified. In this method, historic artifact images of a number of imaging facilities together with the measures for eliminating the respective artifacts are maintained in a data bank. Suitable measures for the elimination of the artifacts can be derived by comparing the artifacts under examination to the historic data. This publication, however, contains no discussion relating to measures for determining the SAR values.